An intervenor is a person or organization that is not an original party to a lawsuit but asks the court for permission to join it because the outcome could affect its own rights or interests. Once the court grants the motion to intervene, the intervenor takes part in the case much like a plaintiff or defendant.
In U.S. federal court, Federal Rule of Civil Procedure 24 sets two paths. Intervention of right must be allowed on a timely motion when a federal statute grants an unconditional right, or when the movant claims an interest in the property or transaction at issue that the case could impair and the existing parties do not adequately represent it. Permissive intervention is up to the judge when the movant shares a common question of law or fact with the case, and the court must consider whether it would unduly delay or prejudice the original parties. The motion must state its grounds and include a pleading setting out the intervenor’s claim or defense.
Source: Federal Rules of Civil Procedure, Rule 24 (Intervention), Cornell Legal Information Institute
Image: Courtroom, Hannibal Federal Building and U.S. Courthouse (2013), U.S. District Court for the Eastern District of Missouri, public domain, via Wikimedia Commons.